This invention relates, generally, to polishing pads used for creating smooth, ultra-flat surfaces on items such as glass, semiconductors, dielectric and metal composites and integrated circuits and, more particularly, to methods for fabricating polishing pads that enable optical end-point detection.
The increasing need to form planar surfaces on a variety of materials has led to the development of process technology known as chemical-mechanical-polishing (CMP). In the CMP process, a substrate to be polished is brought into contact with a polishing pad in the presence of a polishing slurry. As the substrate is brought into frictional contact against the polishing pad, pressure created between the pad and substrate, in conjunction with the action of the polishing slurry, polishes away surface layers of the substrate. The polishing process is assisted by chemical compounds within the polishing slurry that facilitate removal of the material being polished. By carefully selecting the chemical components of the polishing slurry, the polishing process can be made more selective to one type of material than to another. The ability to control the selectivity of a CMP process has led to its increased use for delicate surface applications, such as the fabrication of complex integrated circuits.
A common requirement of all CMP processes is that the substrate be uniformly polished and that the amount of material removed by the polishing process be controlled in a repeatable fashion. Recently, optical techniques have been developed to monitor the polishing process and to determine a process end-point. Typically, the optical end-point detection method involves generating a light beam and reflecting the light beam off of the surface being polished. Because both the surface being polished and the polishing pad are in continuous motion during the polishing process, it is difficult to construct an optical pathway for continuous light transmission. In one technique, an aperture is created in the polishing pad and aligned to an opening in the platen of a CMP apparatus. A stationary light source is positioned in proximity to the platen and opposite to the side of the platen supporting the polishing pad. As the opening in the platen and corresponding aperture in the polishing pad pass over the light source, the light beam emitted by the light source is momentarily reflected by the surface being polished. The reflected optical signals are collected by a detector over time and electrically analyzed to determine a polishing end-point.
The creation of an aperture or window for optical transmission is not straightforward and requires that several processing issues be addressed. For example, a simple hole in the polishing pad would permit polishing slurry to seep through the opening and along the interface between the polishing pad and the platen. Since it is important that the pad be secured to the platen, the incursion of foreign substances between the platen and the polishing pad must be prevented. Further, most polishing apparatus are configured to have electronic systems and supporting mechanical devices below the platen. Accordingly, leakage of polishing slurry and other liquids from the polishing-side of the platen must also be prevented.
Polishing pads are typically composed of two or more overlying layers of different materials. Typically, a polishing pad includes at least a polishing layer overlying a backing layer. Additionally, an adhesive layer is commonly used to adhere the backing layer to the polishing platen. Since the polishing layer and the backing layer are typically composed of different materials, the optical transparency of the materials also differs. Most materials used as a polishing layer are opaque to light over a wavelength range useful for end-point detection. Many of the materials used to construct a backing layer, however, are transparent to light. Accordingly, polishing pads have been fabricated in which sections of the polishing layer are removed and replaced with an optically transparent material. Although this technique is effective at creating an optical pathway, it involves relatively complex processing techniques. In one common process, a section of the polishing layer is removed and an optically transparent material is stitched into the opening. This type of process is time consuming and increases the cost of a polishing pad produced by this method. Accordingly, more efficient process techniques are necessary to fabricate polishing pads having optically transparent regions to enable end-point detection.
The present invention is for a method of fabricating a polishing pad having an optical window. The method includes providing a pad material having a polishing layer overlying a substantially optically transparent layer. A portion of the polishing layer is removed, such that an underlying portion of the optically transparent layer is exposed. Since the underlying substantially optically transparent layer is not pierced when the portion of the polishing layer is removed, the process of the invention provides an optical pathway without producing a leakage path for polishing slurry.
In one embodiment of the invention, the portion of the polishing layer is removed by cutting away the polishing layer using a cutting tool. The cutting tool cuts away a portion of the polishing layer from the substantially optically transparent layer, while the pad material is moved relative to the assembly holding the cutting tool. The cutting tool and pad material are brought into motion relative to one another, such that a precisely defined portion of the polishing layer is removed by the cutting tool. Automation of the cutting process enables the rapid formation of an optical pathway in a polishing pad, and further enables a reduction in the processing time necessary to fabricate such a polishing pad.
In a specific embodiment of the invention, the pad material is placed on a flat cutting surface and a cutting tool is transversely mounted to a carriage assembly. In the fabrication process, the carriage assembly and cutting surface are moved toward one another at substantially a right angle. A rotating disk having a plurality of cutting teeth arranged on the perimeter surface of the disk makes contact with the polishing layer, such that a controlled amount of polishing layer material is removed from the substantially optically transparent layer.